The present invention relates to the field of photolithographic techniques. More specifically, one embodiment of the invention provides an improved lithographic method for fabricating a multitude of electronic devices, including those fabricated on silicon wafers such as those used for linear, power, and other applications. This method is also useful for printing multichip modules, CCDs, diskdrive heads, etc. and smaller flat panel displays. Still more specifically, the present invention relates to a manufacturing method and apparatus wherein an image from a photomask or a reticle (hereafter called a reticle) is imaged onto a photosensitive substrate by scanning means.
Currently there are three types of machines used for printing various patterns in the manufacture of electronic devices. The first type of machine is called a contact or proximity printer. This is the oldest type of machine. The reticle and substrate are in contact or close proximity and are aligned to each other. A flood exposure illuminator illuminates the reticle and thereby exposes the substrate. This machine is relatively low in complexity, and relatively low in cost, but has the large disadvantage that reticles are ruined after a certain number of uses. Manufacturers who use these machines would like to switch to one of the other two types of machines, except the cost is often unacceptable.
The second type of machine is commonly called a step and repeat camera because it moves to a specified location and prints a portion of the photosensitive substrate and then moves to another location and typically prints the same image on another portion of the substrate, repeating this process until the entire substrate is printed. Within this category, there are at least three different object to image magnifications used. Some of the machines manufactured by Nikon, Canon, GCA and others are called reduction steppers, and can print a portion of the final image at 1/5.times. or 1/10.times. magnification. The reticle area printed is equal to the image area divided by the magnification. The state of the art machines can now print image areas of up to 20 mm.times.20 mm at 0.45 or higher numerical aperture. Another manufacturer of step and repeat cameras, Ultratech uses a 1.times. magnification. The Ultratech stepper can print a field of at least 10 mm.times.30 mm at 0.40 numerical aperture. One disadvantage of the 2.times. or any non-1.times. step and repeat technology is the need for precise magnification control. The step and repeat technology requires very precise alignment, which requires a very expensive stage with a precise metrology system. All the step and repeat cameras are complex, expensive and very large, consuming a large area of expensive floor space. The step and repeat cameras have been designed primarily for printing dynamic memory integrated circuits. This market requires resolution down to sub-micron which in turn requires extreme performance from these machines.
The third type of machine is called a scanner. Silicon Valley Group (formerly Perkin-Elmer) and Canon both make scanners that print a 6-inch (150 mm) wide image at 1.times. magnification with resolutions down to about 1 micrometer. These machines are very complex and expensive. See U.S. Pat. Nos. 3,748,015 and 4,293,186. The types of lens system described in the patents consists of two mirrors and putatively forms an image of good resolution in a circle of about half the diameter of the larger mirror. The lens configuration only uses a portion of the high resolution circle, giving these machines the title "Ring field scanner." The reticle object is imaged on the substrate in the form of a circular arc. Printing is accomplished by moving both the reticle object and substrate together, with the reticle object and substrate to be printed on opposing sides of the optical system. Although a symmetric unity or 1.times. magnification lens system does not necessarily have any inherent distortion, the difficulty in manufacturing these optical systems causes them to have a large amount of "manufactured" optical distortion. The Canon MPA-600 and Silicon Valley Group PE700 can typically print an approximately 6-inch wide path.
Both types of machines typically use off-axis alignment techniques which require calibration and the use of more environmentally stable materials. The by-product again is increased costs and decreased productivity.
From the above information it is seen that an improved system and method for fabricating substrates is needed.